Nanofibrillated polysaccharides, such as microfibrillated cellulose (MFC) has many end uses, such as in food, cosmetics, paints, plastics, paper, paperboard, medical products and composites, in which it would be good to be able to dosage microfibrillated cellulose in a dry form so that the original properties of wet micro fibrillated cellulose would be retained. Microfibrillated cellulose used in composites, is typically added in a dry form.
A dispersion of microfibrillated cellulose in water is a gel having pseudoplastic or thixotropic viscosity properties because fibrils are very well dispersed in the matrix (water). On drying, however, the properties of micro fibrillated cellulose are severely changed. It's dispersibility, hydration and viscosity properties may be lost or substantially reduced, depending on the severity of drying. Typically after drying, micro- and nano fibrils are bound together and much less amounts of small scale micro- or nano fibrils can be found via e.g. optical microscope.
When microfibrillated cellulose is dried it would be beneficial that not too much fibril/fibril bonds are formed, so that micro fibrils are free from each other or at least easily liberated when dispersed in a solvent or matrix. At the moment, this can be done by freeze drying or by using solvent exchange type of drying technologies. Also one possibility is to add chemicals such that fibril/fibril contacts are essentially reduced during drying.
When microfibrillated cellulose is used in composites one should ensure that micro fibrils are clearly separated from each other and that micro fibrils are very well dispersed in the matrix.
Conventional drying techniques for drying MFC are currently freeze drying which provides the best quality MFC. However, both the operating and investment costs are high and the process can be difficult to scale up to industrial processing. Spray drying, which on the other hand, can rather easily be scaled up, has high operation costs and feature in which hornification of fibrils is prone to occur.
Typical chemicals used to prevent hornification of cellulose or fibrillated cellulose or cellulose fibrils has been, surface active agents or surface active polymers, carbohydrates and more specifically low molecular weight carbohydrates, starch, CMC and similar derivatives thereof. Processes utilizing chemicals can be up-scaled. However, the costs related to these chemicals can be high, and many of the chemicals are disadvantageous or even harmful in different applications.
The term “hornification” may refer to the stiffening of the polymer structure that occurs in lignocellulosic materials when they are dried or otherwise dewatered. Because of structural changes in the wood pulp fibers upon drying the internal fiber shrinks. Often the fibers needs to be rewetted, or re-suspended in water for practical use and due to these structural changes the original properties, i.e. being in a gel form having pseudoplastic or thixotropic viscosity, is not fully regained. The effect of hornification may be identified in those physical paper or wood pulp properties that are related to hydration or swelling, such as burst or tensile properties. (Hornification—its origin and interpretation in wood pulps, J. M. B. Fernandes Diniz, M. H. Gil, J. A. A. M. Castro, Wood Sci Technol 37 (2004) 489-494).
Further to this nanofibrillated polysaccharides such as MFC often form the basis of, or a part of composites suitable for applications such as plies for paper or paperboards, for use in rheology applications, in paints, foods, pharmaceuticals etc. These composites are often formed by adding a filler, such as precipitated calcium carbonate to the MFC, thus forming a PCC/MFC composite material. The calcium carbonate, or filler material, may be added in a conventional process, such as disclosed in EP2287398 or in a so called in-line process which is disclosed in for instance WO2001/110744. The formation of PCC on fibers may be achieved today in processes where lime milk is mixed in the presence of natural fibers, or dissolved cellulose, cellulose whiskers or fibrils or fibrillated aggregates, or synthetic polymer fibers and carbon dioxide. The precipitation of calcium carbonate then may occur on fibers or into fiber lumen, dissolving pulp, cellulose whiskers or synthetic polymer fibers or mixtures thereof.
Currently, PCC or even nanoPCC can be used with various technologies. The nanoPCC may be provided through methods such as those disclosed in US2009/0022912. It is also known that additives such as PVOH and PAA can be used to control crystal growth and nucleation during the precipitation of calcium carbonate. Such methods are shown in for instance WO 2009/074491 A1.
The problem with the commercially available in-line method is that it is limited to the so called wet end of a paper machine and therefore to very dilute pulp conditions. Typically, pulp composition is preferably below 1.0 wt % or more preferably below 0.5 wt %.
Another disadvantage with the present techniques is that it generates very large particle sizes of the PCC on the fibers and obviously a large fraction is formed in the liquid phase. The formation of large particles is in some cases not preferred since it further affects e.g. optical properties or wettability.
There is therefore a need for an improved process producing composites comprising nanofibrillated polysaccharide and subsequently drying these composites, which is simpler to carry out while yielding a dry or semi-dry MFC composite material, without loss of important re-dispersibility properties, since, if strong hornification or agglomeration occurs during drying the beneficial properties of cellulosic fibrils or fibrillated aggregates are not obtained. It is thus preferable that the dried composite maintains its characteristics when dispersed in other solvents or e.g. polymeric matrices.